The present invention relates to an air-fuel ratio control system for an engine of a motor vehicle, and more particularly for a lean mixture engine.
The lean mixture engine operates on lean mixture at light and middle load and on stoichiometry mixture at heavy load. A feedback air-fuel ratio control is provided for supplying the air-fuel mixture at large air-fuel ratio (lean mixture) or stoichiometric air-fuel ratio in accordance with engine operating conditions.
The feedback system is provided with a lean mixture sensor for sensing the oxygen concentration of the exhaust gas of the engine, the output voltage of which is proportional to the oxygen concentration. In a fuel injection system for the lean mixture engine, a plurality of desired air-fuel ratios are stored in a look-up table in accordance with engine operating conditions. A feedback signal from the lean mixture sensor is compared with a desired air-fuel ratio AFd to produce an error signal. A feedback coefficient K.sub.FB for fuel injection is calculated based on the error signal. On the other hand, an air-fuel ratio coefficient K.sub.AF and a miscellaneous coefficient COEF including a plurality of coefficients based on various operating conditions such as coolant temperature, intake air temperature and others are obtained. Fuel injection time TI of injected fuel is calculated as follows. EQU TI=K.times.K.sub.AF .times.K.sub.FB .times.Q/N.times.COEF
where
K is a correcting coefficient, PA1 Q is intake air flow rate, PA1 N is engine speed.
By injecting fuel during the calculated time, air-fuel ratio is controlled to the desired air-fuel ratio.
When the desired air-fuel ratio AFd varies from one value to another value dependent on changing of engine operating condition during the looking-up the table, the control of air-fuel delays. As a result, the control system oscillates, so that actual air fuel ratio AFa oscillates as shown in FIG. 6.